This application relates generally to fiber-optic communications and more specifically to techniques and devices for routing optical signals to different output ports (or, conversely, routing different spectral bands at the output ports to the input port).
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future.
In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are currently performed by electronicsxe2x80x94typically an electronic SONET/SDH system. However SONET/SDH systems are designed to process only a single optical channel. Multi-wavelength systems would require multiple SONET/SDH systems operating in parallel to process the many optical channels. This makes it difficult and expensive to scale DWDM networks using SONET/SDH technology.
The alternative is an all-optical network. Optical networks designed to operate at the wavelength level are commonly called xe2x80x9cwavelength routing networksxe2x80x9d or xe2x80x9coptical transport networksxe2x80x9d (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable. New types of photonic network elements operating at the wavelength level are required to perform the cross-connect, ADM and other network switching functions. Two of the primary functions are optical add-drop multiplexers (OADM) and wavelength-selective cross-connects (WSXC).
In order to perform wavelength routing functions optically today, the light stream must first be de-multiplexed or filtered into its many individual wavelengths, each on an individual optical fiber. Then each individual wavelength must be directed toward its target fiber using a large array of optical switches commonly called an optical cross-connect (OXC). Finally, all of the wavelengths must be re-multiplexed before continuing on through the destination fiber. This compound process is complex, very expensive, decreases system reliability and complicates system management. The OXC in particular is a technical challenge. A typical 40-80 channel DWDM system will require thousands of switches to fully cross-connect all the wavelengths. Opto-mechanical switches, which offer acceptable optical specifications, are too big, expensive and unreliable for widespread deployment. New integrated solid-state technologies based on new materials are being researched, but are still far from commercial application.
Consequently, the industry is aggressively searching for an all-optical wavelength routing solution that enables cost-effective and reliable implementation of high-wavelength-count systems.
Embodiments of the invention are directed to an optical routing apparatus for directing two optical signals between two input ports and two output ports. Each optical signal follows a path defined by an optical switching arrangement that is adapted to shift among at least two distinct optical configurations. In the first optical configuration, the optical signal provided by the first input port is directed to the first output port and the optical signal provided by the second input port is directed to the second output port. In the second optical configuration, the optical signal provided by the second input port is instead directed to the first output port, while the optical signal provided by the first input port is directed to neither output port. In this manner, a variation of a 2xc3x972 optical switch is provided. Improvements in bandwidth over a 2xc3x972 switch are achieved when wavelength-multiplexed optical signals are used. For example, when such a modified 2xc3x972 switch (i.e. a xe2x80x9c2xc3x972xe2x80x2 switchxe2x80x9d) is used in an add-drop multiplexer configuration, it remains possible to add a signal with wavelength xcex1 and drop a signal with wavelength xcex2 from a trunk signal.
In one embodiment, the optical switching apparatus has two fixed mirrors and two rotatable mirrors. The positions of the two rotatable mirrors are linked so that the first optical configuration is defined by one position for each rotatable mirror and the second optical configuration is defined by a second position for each rotatable mirror. The path of each optical signal of interest includes a reflection off a fixed mirror and off one of the rotatable mirrors. It is preferable that the input and output ports be spaced at the confocal length of one of the optical signals to improve reintegration of the optical signals at the output ports. Where this confocal length is less than the diameter of the optical fibers used to provide the input and output ports, the optical fibers are preferably flattened, such as by shaving a portion of the fibers"" cladding layer.
In another embodiment, the optical switching apparatus uses only a single rotatable mirror with four fixed mirrors. The rotatable mirror has two positions that define the two configurations of the optical switching apparatus. The optical signals are directed so that they are reflected off two of the fixed mirrors and off the rotatable mirror between the fixed-mirror reflections. Accordingly, the rotatable mirror is placed at a focus defined by the arrangement of fixed mirrors. In an alternative embodiment, the four fixed mirrors are substituted with a single mirror having a focus where the rotatable mirror is positioned. Such a single mirror may include a composite mirror or may include a curved mirror, such as a portion of a rotated conic section. The path lengths of the optical pathways are preferably equalized. This is achieved in one embodiment by staggering the input and output ports so that they do not lie in a plane.
In still another embodiment, a rotatable mirror configured to have three positions is used with three fixed mirrors to define a 2xc3x972xe2x80x2 optical switch. In this embodiment, only one of the optical signals provided by the input ports is of interest in any particular configuration. In one exemplary embodiment, when the rotatable mirror is in its first position, the optical signal provided by the first input port is directed to the second output port; when the rotatable mirror is in its second position, the optical signal provided by the first input port is directed to the first output port; and when the rotatable mirror is in its third position, the optical signal provided by the second input port is directed to the second output port. In alternative embodiments, the three fixed mirrors are substituted with a single mirror, which may be composite or curved to define a focus at the position of the rotatable mirror. It is also preferable to equalize path lengths, such as by staggering the input and output ports.